Generally, rotary atomizing head type coating machines have been widely resorted to for coating vehicle bodies or similar work pieces. Shown by way of example in FIGS. 16 and 17 is a prior art rotary atomizing head type coating machine of this sort.
In these figures, indicated at 100 is a rotary atomizing head type coating machine as a whole. The rotary atomizing head type coating machine 100 is largely constituted by a machine cover 101 which is formed in a tubular shape, an air motor 102 which is accommodated in the cover machine 101, a rotational shaft 103 which is passed axially through the air motor 102 and rotationally driven by the latter, and a rotary atomizing head 104 which is mounted on the rotational shaft 103 on the front side of the machine cover 101 and thereby rotated at a high speed, for example, at a speed of 3,000 to 100,000 rpm to atomize and spray paint.
More specifically, as shown in FIG. 17, the rotational shaft 103 is provided with a male screw portion 103A around the circumference of a front end portion which is projected forward of the air motor 102. The rear side of the rotary atomizing head 104 is formed into a rotational shaft mount portion 104A of a tubular shape to receive therein a fore end portion of the rotational shaft 103. The rotational shaft mount portion 104A is provided with a female screw portion 104B on a deep inner peripheral portion for threaded engagement with the male screw portion 103A of the rotational shaft 103. The rotational shaft 103 and the rotary atomizing head 104 are integrally fixed to each other by tightly threading the male screw portion 103A into the female screw portion 104B.
A feed tube 105 is passed through the rotational shaft 103, and the fore end of the feed tube 105 is projected from the rotational shaft 103 and extended into the rotary atomizing head 104. A paint passage 105A and a thinner passage 105B are provided internally of the feed tube 105.
Further, an annular shaping air ring 106 is detachably attached to the front side of the machine cover 101, on the outer peripheral side of the rotary atomizing head 104. In order to control the spray pattern of paint which is sprayed by the rotary atomizing head 104, a large number of air outlet holes 106A are formed in this shaping air ring 106 at intervals in a circumferential direction for spurting shaping air toward sprayed paint particles.
In the case of the rotary atomizing head type coating machine 100 which is constructed in the manner as described above, while the rotary atomizing head 104 is put in high speed rotation by the air motor 102, paint is supplied to the rotary atomizing head 104 through the feed tube 105. Since the rotary atomizing head 104 is in high speed rotation, the supplied paint is atomized into fine particles under the influence of centrifugal force and sprayed forward. The spray pattern of paint particles is controlled by shaping air which is spurted from the shaping air ring 106 while paint particles are sprayed forward for deposition on a work piece.
By the way, according to the above-described prior art rotary atomizing head type coating machine 100, the rotational shaft 103 and the rotary atomizing head 104 are fixed to each other by threaded engagement of the male and female screw portions 103A and 104B. The screw threads of these screw portions 103A and 104B are tapped in the opposite direction with respect to the direction of rotation of the rotary atomizing head 103, more specifically, are tapped as right-turn screws in a case where the rotary atomizing head 103 is put in rotation in a counterclockwise direction as seen arrow a in FIG. 16 (turning leftward when seen from the front side of the rotary atomizing head 104). Therefore, when the rotational speed of the air motor 102 is on the increase, the rotational shaft 103 is tightened relative to the rotary atomizing head 104. On the other hand, when the rotational speed of the air motor 102 is on the decrease, the rotational shaft 103 is loosened relative to the atomizing head 104.
Nevertheless, should a trouble occur to a drive portion of the air motor 102 when it is in high speed rotation, the rotational speed of the air motor 102 could drop abruptly and the rotation itself could be totally stopped. Besides, for the purpose of changing the paint feed rate to the rotary atomizing head 104 or for the purpose of washing the rotary atomizing head 104 prior to a color change, there may often arise a necessity for dropping the rotational speed of the air motor 102 abruptly from about 30,000 rpm to about 10,000 rpm.
In such a case, despite an abrupt drop in rotational speed of the rotational shaft 103, the rotary atomizing head 104 tends to maintain a current rotational speed under the influence of inertial force, as a result acting to loosen the screw portions 103A and 104A, that is to say, to loosen the rotary atomizing head 104 relative to the rotational shaft 103. Therefore, while the air motor 102 is in high speed rotation, an abrupt drop of the air motor speed can lead to loosening and fall-off of the rotary atomizing head 104 from the rotational shaft 103.
Further, the rotary atomizing head 104 is subjected to large centrifugal force while in high speed rotation, and as a result the atomizing head mount portion 104A is spread in radially outward directions, lowering the gripping force of the male and female screw portions 103A and 104B of the rotational shaft 103 and the rotary atomizing head 104. Thus, upon a conspicuous drop in operating speed of the rotational shaft 103 or a sudden stop of the rotational shaft 103, the male and female screw portions 103A and 104B can be loosened until the rotary atomizing head 104 falls off the rotational shaft 103.
If the rotary atomizing head 104 should fall off the rotational shaft 103 during high speed rotation, it would be thrown away and hit against nearby equipments and work pieces, resulting in serious damages not only to the rotary atomizing head 104 itself but also to the nearby equipments and work pieces.
On the other hand, according to another prior art rotary atomizing head type coating machine, an O-ring is fitted on the outer periphery of a fore end portion of the rotational shaft or in the inner periphery of the rotary atomizing head mount portion to mount the rotary atomizing head on the rotational shaft through resilient force of the O-ring (e.g., as disclosed in Japanese Patent Laid-Open No. H11-28391).
However, in the case of the another prior art just mentioned, there is a problem of abrasive damages to the O-ring because the surface of the O-ring is abraded every time the rotary atomizing head is mounted on or dismantled from the rotational shaft. If an O-ring is used in a damaged state, it may no longer be able to stop the rotary atomizing head from falling off the rotational shaft because its force of fixing the rotary atomizing head to the rotational shaft is weakened considerably in the abrasive damage.